This invention relates generally to magnetic angular measurement devices, and, more particularly, to a system for calibrating a magnetic angular measurement device.
Contactless angular measurement devices are known and are typically implemented to determine an angular orientation of a magnetic element on a shaft with respect to at least one magnetoresponsive sensor spaced from the rotating magnetic element, or vice versa. As the magnet rotates, the sensor or sensors generate a signal which may be analyzed to determine the angle of orientation of the magnet relative to the sensor. These measurement devices may be implemented such that the magnetic element is rotatably interconnected with a linearly moving component, such as a suspension unit or control arm of a vehicle. As the component moves vertically, this substantially linear movement may be translated to rotational movement or pivoting of the magnet relative to the sensor. The device may then determine the angle of rotation of the magnet, from which the corresponding displacement of the component on the vehicle is then calculated.
Typically, laser trimming of the sensors or the magnetic element has been used in order to offset errors which may arise due to an internal magnetization field of the sensors being misaligned with an external field of the magnet. Furthermore, the orientation of the magnetic field directions of the sensors relative to the external field directions of the magnetic elements must be known after the device is assembled. After the sensors have been trimmed and the angular measurement device has been fully assembled, the magnet is positionable relative to the sensors such that an internal magnetization vector of one of the sensors is substantially parallel to or otherwise positioned relative to a magnetization vector of the magnet at an initial zero or reference point. While this process is generally effective in manufacturing an accurate sensor for measuring the angular rotation of a magnet, laser trimming adds substantially to the overall cost of the angular measurement devices. Furthermore, any variations in positioning of one sensor relative to another may result in offsets or errors in the output of the device that are unaccounted for during the production of the devices.
An additional concern with these devices is that they typically require a rare earth type of permanent magnet, which is stronger and more magnetically stable than other magnetic materials, such as a common ferrite magnet. These permanent magnets are implemented in order to minimize the effects due to aging of the magnet over the life of the device or due to temperature variations surrounding the device. While this may improve the overall accuracy of the device over time, the rare earth permanent magnets also add to the cost of the devices.
Generally, a particular magnetic element is selected for use in these measurement devices such that the internal magnetic field within the sensors and generated by the magnetic element is substantially greater than any external fields associated with the surroundings of the magnet and sensor. This results in the sensors being saturated by the internal field of the magnetic element when the sensors and magnet are placed substantially near to each other, which further results in the external fields affecting the sensors in a substantially insignificant manner. This avoids the necessity of accounting for offsets due to varying external magnetic fields that is typically required in other magnetic systems, such as vehicle compass systems.
Furthermore, the magnetoresponsive sensors, such as magnetoresistive sensors, typically implemented in these measurement devices are operable in a non-linear mode, such that an output frequency of the sensors is doubled, whereby 180xc2x0 rotation of the magnet generates a full cycle of a sine and cosine output signal of the sensors. In order to improve the accuracy of these non-contacting angular measurement devices, precise manufacturing and assembly of the sensors and the magnetic elements may be necessary. If a single sensor is implemented, the device may be more readily manufactured but is only accurate within a small range of rotation, typically less than approximately plus or minus 15xc2x0. Because the sensors are saturated, orienting two sensors in a predetermined manner, such as 45xc2x0 relative to one another, allows for a measurement of a larger angle of rotation, within a range of 180xc2x0, or plus or minus 90xc2x0 from an initial setting. However, the accuracy of such a device is highly dependent on the manufacturing of the sensors since they must be precisely manufactured and oriented at 45xc2x0 relative to one another, in order to minimize errors in the output of the system. Because such precision is required in manufacturing and assembling these sensors and the magnets, manufacturing irregularities may result in errors in the output of the device.
Therefore, there is a need in the art for a contactless angular measurement device which is relatively low cost yet remains accurate throughout the life of the device. The device should account for manufacturing irregularities, system aging and thermally induced drifts within the sensor device in order to maintain an accurate reading over the life of the device.
The present invention is intended to provide a low cost contactless angular measurement system for use in determining a linear movement of a component of a vehicle by measuring a corresponding rotational movement of a magnetic element that is interconnected with the component.
According to a first aspect of the invention, a vehicular magnetic rotational measurement device for determining a displacement of an element comprises a magnetic element which generates a magnetic field, at least one magnetoresponsive sensor for sensing the magnetic field generated by the magnetic element, and an electronic control for analyzing an output of the sensor and determining a degree of movement of the magnetic element relative to the sensor from the output of the sensor. The magnetic element is spaced from the magnetoresponsive sensor and is movable relative thereto. The sensor is operable to detect a change in the magnetic field of the magnetic element which is associated with a movement of the magnetic element relative to the magnetoresponsive sensor. The electronic control is operable to determine an offset in the output of the sensor and adjusts an output of the electronic control in response to this offset.
According to another aspect of the present invention, a magnetic level sensor is adapted for interconnection with a generally linearly movable component of the vehicle. The level sensor determines an orientation of the component relative to a frame of the vehicle and comprises a magnetic element, at least one magnetoresponsive sensor for sensing a magnetic field generated by the magnetic element, and an electronic control which is operable to analyze an output of the sensor and determine an angle of rotation of the magnetic element or the sensor in response to the output of the sensor. The magnetic element is spaced from the magnetoresponsive sensor and is rotatable relative thereto. The sensor is operable to detect a change in the magnetic field of the magnetic element which is associated with a rotational movement of the magnetic element relative to the sensor. The magnetic element or the sensor is rotatably interconnected to the component of the vehicle such that a generally linear displacement of the component causes a corresponding angular rotation of the magnetic element or the magnetoresponsive sensor. The electronic control is operable to determine an offset in the output of the sensor and adjust an output of the electronic control in response to the offset.
Preferably, the offset is determined in response to an average ordinate value of equal and opposite pairs of data points sampled by the electronic control. The control may initially determine the offset when the vehicle or control is started, or may continuously determine the offset and adjust the output accordingly.
These and other objects, advantages, purposes and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.